Which protein interacts with actin filaments to contract or relax a muscle?

Myosin is the protein that interacts with actin filaments to facilitate the contraction and relaxation of muscle fibers. This interaction forms the basis of the sliding filament model of muscle contraction. In this process, myosin heads bind to specific sites on actin filaments, forming cross-bridges. When myosin heads pivot, they pull the actin filaments toward the center of the sarcomere, thereby shortening the muscle and producing contraction. The energy for this movement is derived from ATP, which is hydrolyzed to provide the necessary energy for the conformational changes in myosin.

Furthermore, the regulation of muscle contraction involves calcium ions, specifically Ca++, which bind to troponin and trigger a conformational change that exposes the binding sites on actin for myosin. However, myosin remains the primary motor protein that executes the contraction at the level of the actin filaments.

Calmodulin is a calcium-binding protein that plays a role in various signaling pathways but is not directly responsible for muscle contraction in striated muscles. Instead, it is more prominent in smooth muscle contraction regulation. ATP, while crucial as an energy source, does not interact directly with actin but is rather used by myosin during the contraction cycle. Therefore